The fault diagnosis of the aerocamera can only judge to system level and the detailed fault elements cannot be diagnosed. To resolve the problem, based on the research of the operation principle of aerocamera auto-focusing system, the fault principle is analyzed according to the measurement and control flow-sheet of aerocamera auto-focusing system. The detailed elements possible producing fault such as stroke switch, high accuracy position sensor, driver, electric motor, RS422 communication and communication request are studied in detail. And the fault diagnosis system is built to complete the detailed elements diagnosis for CCD aerocamera auto-focusing system.